A BLDC motor can be classified into a core type (or radial type) and a coreless type (or axial type), each having a generally cup-shaped (cylindrical) structure, according to whether or not a stator core exists.
A BLDC motor of a core type structure is classified into an internal magnet type of FIG. 2 including a cylindrical stator where coils are wound on a number of protrusions formed on the inner circumferential portion thereof in order to form an electronic magnet structure, and a rotor formed of a cylindrical permanent magnet, and an external magnet type of FIG. 1 including a stator where coils are wound up and down on a number of protrusions formed on the outer circumferential portion thereof, and a rotor formed of a cylindrical permanent magnet on the outer portion of which multiple poles are magnetized.
In the external magnet type BLDC motor as shown in FIG. 1, stator cores 101a around which coils (not shown) are wound are installed on the base of a stator through a supporter, respectively. A cup-shaped rotor 101c is installed through a central rotational shaft 101d, in which the rotor 101c is freely rotated through a bearing installed on the center of the stator, and a cylindrical permanent magnet 101b is attached to the inner circumferential portion of the rotor, to form a predetermined crevice, that is, a gap G with respect to the stator.
When power is applied to the FIG. 1 motor, a magnetic field is created around the coils wound on the stator cores 101a of the stator. Accordingly, a rotor case is rotated by a mutual action with a magnetic flux by the permanent magnet 101b mounted on the rotor 101c. 
In the conventional BLDC motor, a main path of the magnetic flux is a magnetic circuit which forms a closed circuit starting from the permanent magnet and proceeding toward the permanent magnet again and a yoke via the gap and the stator core of the stator.
In the internal magnet type BLDC motor as shown in FIG. 2, a plurality of T-shaped core portions 202c on a stator core around which coils are wound, protrude inwards. Also, the inner sides of the respective core portions form a cylinder of a predetermined diameter. Also, a rotor 202f having a cylindrical permanent magnet including a rotational shaft 202d, or a ring-shaped permanent magnet 202b attached to a cylindrical yoke 202 including a central rotational shaft, is mounted in the inner portion of the cylinder surrounded by the core portions 202c. The internal magnet type BLDC motor rotates in the same manner as that of the external magnet type BLDC motor.
The magnetic circuit in the above-described core type BLDC motor has a symmetrical structure in the radial direction around the rotational shaft. Accordingly, the core type BLDC motor has less axial vibratory noise, and is appropriate for low-speed rotation. Also, since a portion occupied by a gap with respect to the direction of the magnetic path is extremely small, a high magnetic flux density can be obtained even if a low performance magnet is used or the number of magnets is reduced. As a result, a big torque and a high efficiency can be obtained.
However, such a yoke structure causes loss of a yoke material when fabricating a stator. In addition, a special-purpose expensive dedicated winding machine must be used for winding coils around the yoke during mass-production, because the yoke structure is complicated. Also, since a mold for fabricating a stator is expensive, initial investment costs become high.
Meanwhile, in order to improve the shortcomings of the above-described core type BLDC motor, a conventional coreless type BLDC motor proposed by the same applicant as that of the present invention is disclosed in U.S. Pat. No. 5,945,766, as an axial type which is a double rotor type BLDC motor for offsetting axial vibration generated when rotors rotate and simultaneously increasing a torque more than two times.
Between first and second rotors is installed a stator in the above conventional coreless type BLDC motor is installed at a distance by a predetermined gap with respect to the first and second rotors. Around the stator are wound a plurality of bobbin-less coils for applying an electromagnetic force to the first and second rotors in response to an applied DC current. Also, current is applied to the coils so that magnetic fluxes which have identical axial polarities are generated when magnets corresponding to the first and second rotors have opposite polarities, and current is supplied to the first and second rotors so that electromagnetic forces are generated in the opposing directions to each other.
In the case of the axial double rotor type BLDC motor, a stator is disposed in the middle of the first and second rotors, in a manner that a magnetic circuit of a symmetrical structure is formed with respect to the stator and the rotational shaft. Accordingly, because of the first and second rotors and the stator, the number of stator coils are increased two times and the number of field magnets are also increased two times as many as a single rotor structure. Therefore, driving current and magnetic flux density are increased two times. As a result, the axial double rotor type BLDC motor can obtain torque at least two times as much as an identical axial single rotor structure.
The axial coreless type motor has various kinds of merits. However, since a portion occupied by armature windings includes an air gap, a magnetic resistance is high and thus a magnetic flux density is low in comparison with the number of magnets.
In other words, in the case of a magnetic circuit formed by magnets m1 to m4 as shown in FIG. 3, a magnetic resistance is increased very largely at an air gap G formed between magnets m1 and m2 and between magnets m3 and m4, and thus a loss of the magnetic flux occurs. As a result, an efficiency of the motor is lowered.
Also, it requires that an air gap become wider in order to increase the number of turns of armature windings for implementing a high torque motor. For this reason, a magnetic flux density would rather decrease, and thus a motor efficiency further decreases.
Thus, the axial coreless type motor should use higher performance magnets and the more number of magnets, in comparison with a radial core type motor of an equivalent output, and finally may raise production cost.
However, although the axial coreless gap type motor has the above-described various kinds of advantages, it is in a more disadvantageous position than a radial type motor, in view of axial vibration.
Meanwhile, in the case of a radial core type motor, a special-purpose dedicated winding machine should be used for winding coils around the above-described integrated stator core. Accordingly, there have been a number of proposals in order to solve the problems that the initial investment cost becomes very high, and productivity of winding coils around the stator core is low.
For example, in order to separate inner/outer wheels forming a core in an internal magnet type core motor, a stator structure has been altered from an integration type to a division type, to thus facilitate winding of coils, or a coil winding method for cores has been altered without changing an integration type core structure, to thus enhance workability in winding coils.
Meanwhile, an inner/outer double rotor type motor has been proposed for a radial core type motor. However, this motor has only intended to simply increase the number of permanent magnets and utilize an empty space, to thereby enhance a motor output, and the stator structure has had still an integration structure. Thus, the existing problems of the low coil winding workability, the high material loss, the high investment cost for the winding machine, etc., still remain, and the coil windings should be provided doubly at the inner/outer sides of the stator core.
Also, a winding machine for winding coils at the inner side of the core and that for winding coils at the outer side of the core cannot be commonly used. As a result, the investment cost for winding machines increases as much as an increase in the motor output.
In addition to the above-described conventional art, a plurality of division type core structure motors have been proposed for a radial core type motor, in order to increase productivity of winding coils of the stator core and reduce the investment cost for winding machines.